Image processing apparatus and image processing method

ABSTRACT

An image processing apparatus for converting a multiple gradation image into a binary or multivalue image by use of an area gradation method according to a screen cell, which includes: a receiving unit that receives an input pixel value of each pixel of the multiple gradation image and position information on the screen cell; and an output unit that outputs an output pixel value, which is increased or decreased with an increase in the input pixel value, at a pixel position on the screen cell corresponding to the position information, according to the input pixel value and the position information received by the receiving unit.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image processing apparatus whichconverts a multiple gradation image into a binary or multivalue image byuse of an area gradation method according to a screen cell, and moreparticularly to an image processing apparatus, which performs halftoneprocessing to realize fine expression with multiple gradation accordingto dot growth properties that screen cell dots increase or decrease withan increase in gradation degree, and an image processing method.

2. Description of the Related Art

For example, for halftone processing (halftoning) of a multiplegradation image of a laser printer, an area gradation method isextensively used to array prescribed screen cells to form a multiplegradation image and to largely grow dots in a prescribed order with anincrease in gradation value.

Incidentally, in a device with poor reproducibility of isolated dotssuch as a laser printer, it is possible to improve reproducibility of alow gradation portion and to improve the definition of intermediate andhigh gradation portions by configuring a screen cell to have large dotswith a small line number in the low gradation portion and a screen cellto have fine dots with a large line number in the intermediate and highgradation portions.

FIG. 13A through 13F are diagrams showing a screen cell used forconventional halftone processing and dot growth thereof.

FIG. 13A through 13C show that a screen cell 60 with a small line number[FIG. 13A] is used, individual dots in the screen cell 60 grow in aprescribed order with an increase in the gradation value of individualpixels of multiple gradation image data to be converted, and when a dotshape with a prescribed gradation value is determined [FIG. 13B], thepertinent dot shape is remained as it is when a gradation value ishigher than it, and the dots further grow [FIG. 13C].

Similarly, FIG. 13D through 13F show that a screen cell 70 with a largeline number [FIG. 13D] is used, and when a dot shape with a prescribedgradation value is determined [FIG. 13E] in a process of growingindividual dots in the screen cell 70 according to a prescribed orderwith an increase in the gradation value of individual pixels, the dotshape is remained as it is when a gradation value is higher than it, andthe dots further grow [FIG. 13F].

Thus, according to the conventional known halftone processingtechnology, the dots in the screen cell grow according to the prescribedorder, and when a dot shape with a prescribed gradation value isdetermined, the dot shape is remained as it is when the gradation valueis higher than it, and the dots grow furthermore.

According to the above method, the dot shape which has become largecannot be made smaller. Therefore, it is impossible to switch a screenline number according to a prescribed gradation value by using onescreen cell in such a way that the low gradation portion is based on asmall line number screen, and the intermediate and high gradationportions are based on a large line number screen.

Therefore, there are developed various technologies that a screen cellwith plural line numbers is provided in advance and switched.

Japanese Patent Laid-Open Publication No. 2000-71439 describes atechnology that in a multivalue printer having plural kinds of dots suchas small dots and large dots, such as an inkjet printer, has a highlightportion interlocked with the small dots and made to come interlockingwith the large dots as it becomes thick.

Japanese Patent Application Laid-Open No. 11-331562 discloses atechnology that a screen is switched between a small line number and alarge line number depending on the image properties (edge/non-edge).

Japanese Patent Application Laid-Open No. 11-331562, No. 7-254985, No.7-254986, No. 7-283941, No. 8-114965, No. 8-125863 and No. 8-156329disclose technologies that to improve reproducibility of a highlightportion, pulse width modulation is used for writing, and ditherprocessing with 2 dots weighted in a main scanning direction isperformed to stabilize the highlight portion by reproduction with asmall line number.

Especially, Japanese Patent Application Laid-Open No. 8-156329 disclosesa technology that it is judged whether a kind of image density signal isa line image or a natural image, and an optimum recording line number isselected depending on the kind.

According to the existing technologies described in Japanese PatentApplication Laid-Open No. 2000-71439, No. 11-331562, No. 7-254985, No.7-254986, No. 7-283941, No. 8-114965, No. 8-125863 and No. 8-156329described above, plural types of screen cells are provided in advance,the screen cells are switched depending on the image properties(characters/photos, edges/non-edges and the like) or depending on lowdensity or high density.

Therefore, it is necessary to provide previously the plural types ofscreen cells, and it is necessary to have a mechanism of switching theindividual screen cells.

According to the above conventional method, the screen cell numbers tobe switched are variable depending on the printer properties and anobject, and there may be a problem that flexible compliance cannot bemade by a prescribed number of screen cells.

An increase in the types of screen cell numbers subject to switchingincreases the storage capacity of the screen cell and makes theswitching control complex, resulting in increasing the apparatus cost.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above circumstancesand provides an image processing apparatus, which does not requireplural screen cells or a switching function thereof and can switchflexibly a screen structure, and an image processing method.

An aspect of the present invention provides an image processingapparatus for converting a multiple gradation image into a binary ormultivalue image by use of an area gradation method according to ascreen cell, which includes: a receiving unit that receives an inputpixel value of each pixel of the multiple gradation image and positioninformation on the screen cell; and an output unit that outputs anoutput pixel value, which is increased or decreased with an increase inthe input pixel value, at a pixel position on the screen cellcorresponding to the position information, according to the input pixelvalue and the position information received by the receiving unit.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will be described in detail basedon the following figures, wherein:

FIG. 1 is a block diagram showing a functional structure of an imageprocessing apparatus according to the present invention;

FIG. 2 is a block diagram showing a functional structure of the halftoneprocessing section of FIG. 1;

FIG. 3 is a diagram showing a structure of a screen cell used forhalftone processing;

FIG. 4 is a flow chart showing halftone processing by a halftoneprocessing section;

FIG. 5 is a conceptual view showing an operation of halftone processingby the halftone processing section;

FIG. 6 is a conceptual view showing a state of expanding conversion dataafter the halftone processing;

FIG. 7A through FIG. 7D are timing charts showing a laser pulse outputmode of a laser exposure device;

FIG. 8 is a table showing data contents of a lookup table used in afirst embodiment;

FIG. 9 is a diagram showing a table data change curve of a cell withcell number 36 of the first embodiment;

FIG. 10A through FIG. 10D are diagrams showing screen dot growthaccording to the first embodiment;

FIG. 11 is a table showing data contents of a lookup table used in asecond embodiment;

FIG. 12A through FIG. 12F are diagrams showing screen dot growthaccording to the second embodiment; and

FIG. 13A through FIG. 13F are diagrams showing screen cells used forconventional halftone processing and dot growth thereof

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described in detail withreference to the accompanying figures.

FIG. 1 is a block diagram showing a functional structure of the imageprocessing apparatus according to the present invention.

This image processing apparatus 10 is, for example, a laser printer andprovided with a data input section 11, a halftone processing section 12,an image processing section 13 and an image forming section 14.

For example, the data input section 11 takes image data, which isinstructed to be printed, from an unshown client PC and inputs to thehalftone processing section 12.

The image processing apparatus of the present invention is assumed to bean 8-bit system, and the image data which is input to the halftoneprocessing section 12 is intermediate gradation (multiple gradation)image data having a gradation value (pixel value) of, for example, anyof 256 gradations having values of 0 to 255 for individual pixels.

The halftone processing section 12 performs halftone processing toconvert image data (multiple gradation image data) which is input fromthe data input section 11 to, for example, a multivalue image by use ofan area gradation method according to the screen cell.

FIG. 2 is a block diagram showing a functional structure of the halftoneprocessing section 12 of the image processing apparatus 10 shown in FIG.1.

As shown in FIG. 2, the halftone processing section 12 is provided witha setting portion 121 which sets a size of a screen cell having a sizeof n×n corresponding to a pixel position of input image (multiplegradation image) data and information about how to array the screen cellin input image data to be converted and processes to calculate theposition of a current screen cell according to the set information; anaddress generation portion 122 which generates an address of a lookuptable 123 according to the screen cell position calculated by thesetting portion 121 and an input pixel value; the lookup table 123 inwhich an output pixel value is stored in correspondence with the inputpixel value; and an output processing portion 124 which reads the outputpixel value corresponding to the input pixel value from the lookup table123 for each pixel according to the address generated (converted) by theaddress generation portion 122 and outputs to the image processingsection 13.

In the construction of the halftone processing section 12, the settingportion 121 sets a screen cell size of a screen cell 50 which iscomprised of an 8×8 cell arrangement as shown in, for example, FIG. 3and information about how the screen cell 50 is arrayed in input imagedata which is to be converted.

In FIG. 3, numerical values described in the 64 individual cellsconfiguring the screen cell 50 are cell numbers corresponding to theindividual cells.

FIG. 4 is a flow chart showing halftone processing by the halftoneprocessing section 12 of the image processing apparatus 10 according tothe present invention.

When multiple gradation image data of document information to be printedis input from a client PC to the halftone processing section 12, theaddress generation portion 122 takes the pixel value of a first pixel ofthe input image data (step S101) and also obtains information about thepixel position of the pertinent pixel and the cell size of the screencell 50 from the setting portion 121 (step S102), generates an addressof the lookup table 123 according to the obtained information and thepixel value (input pixel value) of the pixel (step S103) and gives theaddress to the output processing portion 124.

The output processing portion 124 refers to the lookup table 123according to the address which is generated (converted) by the addressgeneration portion 122 (step S104), reads an output pixel valuecorresponding to the address from the lookup table 123 and outputs asthe output pixel value to the image processing section 13 (step S105).

When the conversion of the first pixel of the input image data into theoutput pixel value is completed, the address generation portion 122checks whether or not there is a next pixel (step S106). If there is anext pixel (YES in step S106), the address conversion processing isperformed according to the pixel position of the pixel and the inputpixel value through steps S101 to S103. The converted address is sent tothe output processing portion 124, and an output pixel valuecorresponding to the address (input pixel value of the pixel) is readfrom the lookup table 123 and output to the image processing section 13(steps S104 to S105).

The process from step S101 to step S105 is continued until it is judgedthat there is a next pixel (YES in step S106). If it is judged duringthe above process that there is not a next pixel (NO in step S106), itis judged that the processing of all the pixels is completed, and theseries of halftone processing (conversion processing from the inputmultivalue image to an output multivalue image) is terminated.

FIG. 5 is a conceptual view showing an operation of halftone processingby the halftone processing section 12.

In FIG. 5, the halftone processing section 12 receives input of imagedata (aggregation of pixels having gradation values 0 to 255) ofdocuments to be printed from the input side, for example, a client PC,takes a first pixel (a11) of the input image data, determines an addressof the lookup table 123 corresponding to the pixel (a11) and reads apertinent output pixel value from the lookup table 123 according to theaddress. Here, the screen cell 50 is arrayed in the region of all thepixels of the pertinent input image data to perform halftone processing,and output pixel values corresponding to the 64 pixels are readsequentially with the screen cell 50 arrayed in the first 64 pixels atthat time.

Specifically, the output pixel values of the individual cell numbers ofthe screen cell 50 are stored in correspondence with the pluraldifferent input pixel values in the lookup table 123, and one outputpixel value is read in correspondence with the input pixel value of onepixel a11 because a screen cell with an 8×8 cell arrangement (see FIG.3) is used as the screen cell 50 in the present invention.

In FIG. 5, the numerical values in the individual cells of the screencell 50 represent the cell number of the pertinent cell in the upperlevel and the output pixel value of the pertinent cell number in thelower level.

But, in the example shown in FIG. 5, the output pixel values of theindividual cells in the screen cell 50 do not correspond to the inputpixel values at that time.

It is exemplified that as the value of an output pixel valuecorresponding to the input pixel value of one pixel, there might beoutput pixel values for 32 gradations having 0 to 255 at 8 intervals. Inpractice, for example, at the time of processing the input pixel all,the output pixel value corresponding to the pertinent input pixel value(8 in this example) is arranged in the corresponding cell in the screencell 50.

Subsequently, in the halftone processing section 12, processing iscontinued such that the individual pixels are sequentially taken up tothe final pixel of the input image data as described above, the outputpixel values corresponding to the input pixel values of the pixels aredetermined from the lookup table 123, and they are sequentially sent tothe image processing section 13.

In response to the above processing, the image processing section 13sequentially takes in the output pixel values (0, 8, 16, . . . , 255) ofthe individual pixels which are output from the halftone processingsection 12, combines the output pixel values of the individual pixels inthe cell arrangement (8×8) of the screen cell 50 as shown in, forexample, FIG. 6 and expands in regions corresponding to the individualpixels in an image memory 131.

Thus, after the halftone processing section 12 completes reading of theoutput pixel values of all the pixels of the input image data, theoutput pixel values of the individual pixels of all the pixels areexpanded in a cell arrangement (8×8) of the screen cell 50 on the imagememory 131. As a result, the halftone processing to array the screencell 50 for each pixel in all regions of the input image data and toconvert the individual pixels to the multiple gradation image iscompleted.

When the expansion of the output pixel values (conversion to outputimage data) corresponding to all the pixels of the input image data bythe halftone processing is completed, the image processing section 13reads and scans the output pixel values expanded in the image memory 131line by line as indicated by the arrow shown in, for example, FIG. 6,and transfers sequentially to the image forming section 14.

In the image forming section 14, according to the output pixel values(0, 8, 16, . . . , 255) transferred from the image processing section13, an intermediate gradation image corresponding to the input imagedata is printed out on a recording medium through a series ofelectrophotographic process in that exposing and scanning are performedon each line up to the final line while adjusting laser intensity (pulsewidth) of a laser exposure device to form an electrostatic latent imagefor one page on a photoconductor, the electrostatic latent image isdeveloped (toner imaging), and the toner image is transferred to arecording medium, and heating for fixation is performed.

Through the above-described series of processing, the pixel value(output pixel value) of the image data converted by halftone processingperformed by the halftone processing section 12 on the input image datais replaced with a laser pulse width in one pixel of the laser exposuredevice when the image processing apparatus 10 of the present inventionis a laser printer.

Here, the output pixel value, which is input from the image processingsection 13 to the image forming section 14, is obtained by converting inthe halftone processing section 12 an input pixel value having any ofvalues 0 to 255 to any of values of 32 gradations of 0, 8, 16, . . . ,255 (from 8 to 248 at 8 intervals: 255 comes next to 248) for each pixelby use of the screen cell 50 (see FIG. 5).

Thus, the image forming section 14 receives the above-described outputpixel values (0, 8, 16, . . . , 255) transferred from the imageprocessing section 13 and outputs a laser beam having a pulse widthcorresponding to the pertinent individual output pixel values from thelaser exposure device to perform exposure and scanning according to thelaser pulse output timing charts shown in, for example, FIG. 7A throughFIG. 7D.

As apparent from FIG. 7A through FIG. 7D, when the output pixel value iszero (0) [FIG. 7A], the laser beam is not output (laser extinguished),and when the output pixel value is 255 [FIG. 7D], the laser beam with amaximum pulse width (=t1) is output (laser fully lighted). And, for theoutput pixel values (8, 16, . . . , 248) during the above process, alaser beam is output with pulse widths (t11, t12, . . . ) according tothe individual pixel values to perform exposure and scanning asexemplified in FIG. 7B and FIG. 7C.

By the above exposure and scanning, the output pulse width of the laserbeam is reflected to the printing density of the pixel, and the printingdensity becomes dark as the pulse width becomes longer.

Thus, the converted image data expanded in the image memory 131 shown inFIG. 6 has its individual pixels printed as dot patterns (see FIG. 10Athrough FIG. 10D and FIG. 12A through FIG. 12F) which are filled in at adensity (white when output pixel value=0, total block when output pixelvalue=255, and high density as a numerical value is larger between thevalues 0 and 255) corresponding to the output pixel value. As a result,the input multivalue image can be reproduced as a multivalue image byuse of the area gradation method according to the screen cell 50.

As described above, the image processing apparatus 10 of the presentinvention has a halftone processing function which converts inputmultivalue image data having pixel values (gradations) 0 to 255 tomultivalue image data of an 8×8 arrangement expressed in 32 gradationsfor each pixel by the halftone processing section 12 by use of the areagradation method according to the screen cell 50.

Especially, the halftone processing section 12 in the image processingapparatus 10 of the present invention has a feature provided with ahalftone processing function to convert input image data into areagradation data by using one screen cell 50 with the number of dots to befilled in the screen cell 50 not following a simple increase based onarbitrary screen line numbers with respect to an increase in thegradation of each pixel of the input image data but following the dotgrowth properties to increase or decrease with respect to the increasein the gradation of each pixel of the input image data.

The halftone processing function according to the present invention willbe described in detail with reference to individual embodiments.

Embodiment 1

The image processing apparatus 10 according to the first embodiment isprovided with the halftone processing section 12 which is comprised ofthe functional structure shown in the block diagram of FIG. 2.

In the structure of the halftone processing section 12 shown in FIG. 2,the setting portion 121 sets a pixel position, a cell size and the likerelated to the operation of the screen cell 50 which is comprised of the8×8 cell (having cell numbers 1 to 64) arrangement shown in FIG. 3.

In this embodiment, for the lookup table 123 in the construction shownin FIG. 2, for example, a lookup table 123 a which stores the datacontents shown in FIG. 8 is used on the assumption that theabove-described screen cell 50 is used.

In the lookup table 123 a, the output pixel value of each of cellnumbers 1 to 64 (FIG. 8 shows only cell numbers 1, 2, 10, 16 and 36 butall of cell numbers 1 to 64 are stored) in the screen cell 50 is storedin correspondence with each of the input pixel values (input values).

FIG. 8 shows input pixel values of only 0 to 129, but in practice,values 129 through 255 are stored.

Meanwhile, the address generation portion 122 holds an inputvalue/address conversion table showing a correspondence relationshipbetween the individual input pixel values and the storage addresses ofthe individual input pixel values in the lookup table 123 a.

Thus, the address generation portion 122 extracts sequentially the inputpixel values of the individual pixels from the input image data,determines the addresses corresponding to the extracted input pixelvalues from the above-described input value/address conversion table,converts the input pixel values into the corresponding addresses andnotifies to the output processing portion 124.

In the output processing portion 124, the addresses converted by theaddress generation portion 122 are used as a key to retrieve the lookuptable 123 a, and the output pixel values of the individual cells withcell numbers 1 to 64 corresponding to the input pixel values stored inthe addresses are read and output.

Here, as to the data contents of the lookup table 123 a shown in FIG. 8,a change in the output pixel value of each of the individual cellnumbers corresponding to the change in the input pixel value isobserved.

As to, for example, the cell with cell number 1 in the lookup table 123a, when the input pixel value is 0 to 7, the output pixel valueincreases sequentially from 0 to 244224 at 32 intervals, and when theinput pixel value is between 8 and 129, the output pixel value is keptat 255.

As to the cell with cell number 2, when the input pixel value is between0 and 56, the output pixel value is 0, when the input pixel value isbetween 57 and 64, the output pixel value increases from 32 to 255, andwhen the input pixel value is 65 or more, the output pixel value returnsto zero (=0). After that, the output pixel value is kept at zero.

As to the cell with cell number 10, when the input pixel value isbetween 0 and 64, the output pixel value is 0, when the input pixelvalue is between 65 and 95, the output pixel value increases from 8 to248 at 8 intervals, and when the input pixel value is 96 or more, theoutput pixel value is kept at 255.

As to the cell with cell number 16, when the input pixel value isbetween 0 and 48, the output pixel value is 0, when the input pixelvalue is between 49 and 55, the output pixel value increases from 32 to224 at 32 intervals, and when the input pixel value is between 56 and64, the output pixel value is kept at 255. After that, when the inputpixel value becomes 65 or more, the output pixel value returns to 0, andthen, the output pixel value is kept at 0 until the input pixel valuebecomes 129.

As to the cell with cell number 36, when the input pixel value isbetween 0 and 24, the output pixel value is 0, when the input pixelvalue is between 25 and 31, the output pixel value increases from 32 to224 at 32 intervals, when the input pixel value is between 32 and 64,the output pixel value is kept at 255, and when the input pixel value is65, the output pixel value returns to 0. After that, the output pixelvalue is kept at 0 until the input pixel value becomes 96, and when theinput pixel value is between 97 and 127, the output pixel valueincreases from 8 to 248 at 8 intervals, and when the input pixel valueis 128 and 129, the output pixel value becomes 255.

FIG. 9 is a diagram showing a table data change curve of the cell withcell number 36. As apparent from FIG. 9, the output pixel value of thecell with the cell number 36 monotonically increases (input pixelvalue=24 to 32) from zero (=0) with an increase in the input pixelvalue, then returns to zero (when input pixel value=64) and changesagain to monotonically increase (input pixel value=96 to 129).

As exemplified above about the changes in the output pixel values ofseveral cells corresponding to the changes in the input pixel values,the lookup table 123 a of the image processing apparatus 10 according tothe first embodiment includes an address of a data region, in which theoutput pixel values corresponding to the input pixel values are stored,to increase and decrease the corresponding output pixel value with theincrease in the input pixel value.

Especially, the output pixel value corresponding to the input pixelvalue is stored in correspondence with the address to monotonicallyincrease from zero (=0) with the increase in the input pixel value, thento return to zero and then to monotonically increase again like the cellwith the cell number 36 or the like.

Then, the screen cell dot growth according to the halftone processing bythe halftone processing section 12 by use of the lookup table 123 ahaving the above data structure will be described with reference to FIG.10A through FIG. 10D.

In FIG. 10A through FIG. 10D, FIG. 10A shows the dot pattern of thescreen cell 50 when the input pixel value is 52 [at this time, an arearatio of lighted cells (cells with output pixel values other than zero)with respect to the entire cell region in the screen cell 50 is20.3125%].

Similarly, FIG. 10B shows the dot pattern of the screen cell 50 when theinput pixel value is 64 (area ratio=25%), FIG. 10C shows the dot patternof the screen cell 50 when the input pixel value is 65 (area ratio=25%),and FIG. 10D shows the dot pattern of the screen cell 50 when the inputpixel value is 112 (area ratio=43.75%).

In FIG. 10A through FIG. 10D, when the input pixel value is 52, theoutput pixel values read from the lookup table 123 a (see AD52 of FIG.8) are 255 for the individual cells with cell numbers 1 and 36, 0 forthe individual cells with cell numbers 2 and 10, 128 for the cell withcell number 16, . . . . As a result, where the output pixel values readfor all the cells are arranged in the related cell regions of the screencell 50 (in practice, corresponding to a case that printing is effectedby laser output with a pulse width corresponding to the pertinent inputpixel value according to the pertinent arrangement), the dot patternbecomes as shown in FIG. 10A.

In FIG. 10A, for example, the individual cells with cell numbers 1 and36 are lit (fully lighted) in correspondence with the read output pixelvalue=255, and the individual cells with cell numbers 2 and 10 are in anon-lighted state in correspondence with the read output pixel value=0.And, the cell with cell number 16 is lit with a pulse width [pulse widthshorter than when fully lighted: corresponding to the output pixelvalue] in correspondence with the read output pixel value=128.

Similarly, cells with numbers 8, 16, . . . , 248 read as output pixelvalues fall in a lit state with a pulse width corresponding to thepertinent output pixel values.

In FIG. 10A through FIG. 10D, when the input pixel is 64, the outputpixel values read from the lookup table 123 a (see AD64 of FIG. 8) are255 for the individual cells with cell numbers 1, 2, 16 and 36, 0 forthe cell with cell number 10, . . . . As a result, the screen cell 50has a dot pattern as shown in FIG. 10B.

In FIG. 10B, for example, the individual cells with cell numbers 1, 2,16 and 36 are lit (fully lighted) in correspondence with the read outputpixel value 255, and the cell with the cell number 10 is extinguished incorrespondence with the read output pixel value=0.

In FIG. 10A through FIG. 10D, when the input pixel value is 65, outputpixel values 255, 0, 8, 0, 0, . . . are read for the individual cellswith cell numbers 1, 2, 10, 16, 36, . . . from the lookup table 123 a(see AD65 of FIG. 8). As a result, the screen cell 50 has a dot patternas shown in FIG. 10C.

In FIG. 10C, for example, the cell with cell number 1 is lit (fullylighted) in correspondence with the read output pixel value 255, theindividual cells with cell numbers 2, 16 and 36 are extinguished incorrespondence with the read output pixel value=0, and the cell withcell number 10 is lit with a pulse width in correspondence with the readoutput pixel value=8.

In FIG. 10A through FIG. 10D, when the input pixel value is 112, outputpixel values 255, 0, 255, 0, 128, . . . are read for the individualcells with cell numbers 1, 2, 10, 16, 36, . . . from the lookup table123 a (see AD 112 of FIG. 8). As a result, the screen cell 50 has a dotpattern as shown in FIG. 10D.

In FIG. 10D, for example, the individual cells with cell numbers 1 and10 are lit (fully lighted) in correspondence with the read output pixelvalue 255, the cells with cell numbers 2 and 16 are extinguished incorrespondence with the read output pixel value=0, and the cells withthe cell number 36 is lit with a pulse width in correspondence with theread output pixel value=128.

Here, the growth of the screen cell dot by use of the lookup table 123 ahaving the data structure shown in FIG. 8 is verified through thecomparison of the individual screen dot patterns of FIG. 10A throughFIG. 10D.

In FIG. 10A through FIG. 10D, when attention is focused on the cell withcell number 2 having an output pixel value increased at the time of achange from input pixel value=52 [FIG. 10A] to input pixel value=64[FIG. 10B], the cell is extinguished (output value=0) when changed frominput pixel value=64 [FIG. 10B] to input pixel value=65 [FIG. 10C].

At this time (when changed from input pixel value 64 to 65), theindividual cells (dots) with cell numbers 8, 16, 21, 27, 28, 36, 38, 44,49 and 63, which were in a lit state when input pixel value=64, areextinguished in the same manner as the cell with the cell number 2 [seeFIG. 10C].

Instead, for the individual cells with cell numbers 5, 11, 15, 19, 23,25, 33, 43, 47, 51, 55 and 61, the cells, which were extinguished(output value=0) when input pixel value=64 [see FIG. 10B], are lit(output value=255) when input pixel value=65 [see FIG. 10C].

Thus, when the input pixel value is changed from 64 to 65, the shape ofthe screen cell 50 is switched from a state of small line number (linenumber=5) in FIG. 10B to a state of large line number (line number=10)in FIG. 10C.

Then, at the time of changing from input pixel value=65 [FIG. 10C] toinput pixel value=112 [FIG. 10D], the output values of the individualcells with cell numbers 4, 8, 10, 14, 18, 22, 28, 32, 36, 40, 42, 46,50, 54, 60 and 64 are sequentially increased with the increase of theinput pixel value on the basis of the dot growth of the large linenumber screen so to have the dot pattern as shown in FIG. 10D.

Thus, an operation capable of switching the screen shape from a smallline number to a large line number with the input pixel value 64 as theborderline can be realized by adopting the lookup table 123 a as shownin FIG. 8 in which an output pixel value corresponding to the inputpixel value is stored so that screen cell numbers having acharacteristic, that a corresponding output pixel value increases anddecreases with the increase in the input pixel value, are intermingled,e.g. the output pixel value monotonically increases from zero with theincrease in the input pixel value, returns to zero when the input pixelvalue exceeds 64 and monotonically increases again with the increase inthe input pixel value.

According to the construction of the embodiment described above, thescreen structure can be switched from a small line number to a largeline number by use of one screen cell 50 by controlling lighting andextinguishing of the individual cells of the screen cell 50, and animage having a different screen line number between a low gradationportion and a high gradation portion can be formed by use of a singlelookup table.

Specifically, a low gradation area (area with input pixel values 0 to 64in this embodiment) with a prescribed input pixel value or belowprovides stable reproduction by a small line number screen. As a result,gradation becomes good, and intermediate and high gradation areas (areawith an input pixel value 65 or more) can improve the definition of areproduced image by use of a large line number screen.

In the above-described embodiment, switching between two kinds of screenline numbers is exemplified, but switching among three kinds or more ofscreen line numbers can be made by the same manner.

According to the above description, it is exemplified that an inputmultiple gradation image is converted to a multivalue image by use of anarea gradation method according to the screen cell, but it is alsopossible to convert to a binary image.

In such a case, for the output pixel values 0 to 255 which are set incorrespondence with the individual screen cell numbers in the lookuptable 123 a shown in, for example, FIG. 8, a replace/read processingfunction which performs reading and outputting with 0 to less than 129replaced by “0” and 129 or more replaced by “1” may be added.

When one screen cell is used to switch the screen line number in thisembodiment, it is set to switch the screen [FIG. 10B] of line number 5to the screen [FIG. 10C] of line number 10 with a prescribed input pixelvalue as the borderline as shown in, for example, FIG. 10A through FIG.10D. In addition, it is also possible to operate so that the screen linenumber increases continuously with the increase in the input pixel valueas indicated by line number 5→line number 6→line number 7 . . . , inconformity with, for example, a low gradation area, an intermediategradation area, a high gradation area and the like.

In the above case, a lookup table in which output pixel valuescorresponding to input pixel values are stored may be adopted instead ofthe lookup table 123 a used in the first embodiment, so that the screenline number increases continuously with the increase in the input pixelvalue on the assumption that the output pixel value corresponding to theinput pixel value is stored (data setting here is the same as in thelookup table 123 a) so as to increase or decrease the correspondingoutput pixel value with the increase in the input pixel value.

Embodiment 2

The image processing apparatus 10 according to the second embodiment isprovided with the halftone processing section 12 which is comprised ofthe functional structure shown in the block diagram of FIG. 2. In thestructure, the setting portion 121 keeps the screen line number at aprescribed level with the increase of the input pixel value as describedin detail later with respect to the screen cell 50 (see FIG. 3: the cellstructure is the same as in the first embodiment) but performs settingof a pixel position, a cell size and the like applied to an operationsuch that an image with different dot positions can be formed.

In this embodiment, for the lookup table 123 in the construction shownin FIG. 2, the lookup table 123 b, in which the data contents shown in,for example, FIG. 11 are stored, is adopted.

In the lookup table 123 b, the output pixel values for the cell numbers1 to 64 (only cell numbers 2, 7, 14, 15, 22, 23, 27 and 30 are shown inFIG. 11, but all cell numbers 1 through 64 are stored in practice) ofthe screen cell 50 are stored in correspondence with the individualinput pixel values (input values).

As to the input pixel values, only 64 through 96 are shown in FIG. 12Athrough FIG. 12F, but values 0 through 255 are stored in practice.

Meanwhile, the address generation portion 122 holds an inputvalue/address conversion table which shows a corresponding relationshipof the individual input pixel values and the storage addresses of theindividual input pixel values in the lookup table 123 b, extractssequentially the input pixel values of the individual pixels from theinput image data, determines the addresses corresponding to theextracted input pixel values from the input value/address conversiontable, converts the pertinent input pixel values to correspondingaddresses, and notifies to the output processing portion 124.

The output processing portion 124 retrieves the lookup table 123 b usingthe addresses converted by the address generation portion 122 as a key,and reads and outputs the output pixel values of the individual cellswith cell numbers 1 through 64 corresponding to the input pixel values(64 through 96) stored in the addresses.

Here, as to the data contents of the lookup table 123 b in FIG. 11, achange in the output pixel value of each of the individual cell numberswith respect to the change in the input pixel value is observed.

In the lookup table 123 b, for example, as to the cell with the cellnumber 2, the output pixel value is kept at 255 when the input pixelvalue is between 64 and 72, the output pixel value decreasessequentially from 224 to 0 at 32 intervals when the input pixel value isbetween 73 and 80, and then the output pixel value is kept at 0 when theinput pixel value is between 81 and 96.

And, as to the cells with cell numbers 7 and 30, the output pixel valueincreases sequentially from 0 to 224 at 32 intervals when the inputpixel value is between 64 and 71, and then the output pixel value iskept at 255 when the input pixel value is between 72 and 96.

As to the cell with the cell number 14, the output pixel value is 0 whenthe input pixel value is between 64 and 80, the output pixel valueincreases from 32 to 224 at 32 intervals when the input pixel value isbetween 81 and 87, and the output pixel value is kept at 255 when theinput pixel value becomes 88 or more.

As to the cells with cell numbers 15 and 22, the output pixel value is 0when the input pixel value is between 64 and 72, the output pixel valueincreases from 32 to 224 at 32 intervals when the input pixel value isbetween 73 and 79, and the output pixel value is kept at 255 when theinput pixel value becomes 80 or more.

As to the cell with cell number 23, the output pixel value is 0 when theinput pixel value is between 64 and 88, the output pixel value increasesfrom 32 to 224 at 32 intervals when the input pixel value is between 89and 95, and the output pixel value becomes 255 when the input pixelvalue is 96.

And, as to the cell with cell number 27, the output pixel value is 255when the input pixel value is 64, the output pixel value decreases from224 to 0 at 32 intervals when the input pixel value is between 65 and72, and the output pixel value is kept at 0 when the input pixel valueis 73 or more.

As exemplified above about the changes in the output pixel values ofseveral cells corresponding to the changes in the input pixel values,the lookup table 123 b of the image processing apparatus 10 according tothe second embodiment includes the addresses of the data regions inwhich the output pixel values corresponding to the input pixel valuesare stored to increase and decrease the corresponding output pixelvalues with the increase in the input pixel values. Especially, thescreen line number is kept at a prescribed level with the increase inthe input pixel value, and the output pixel values corresponding to theinput pixel values are stored in correspondence with the addresses so toform an image with a different dot position.

Then, the screen cell dot growth according to the halftone processing bythe halftone processing section 12 by use of the lookup table 123 bhaving the above data structure will be described with reference to FIG.12A through FIG. 12F.

In FIG. 12A through FIG. 12F, FIG. 12A shows the dot pattern of thescreen cell 50 when the input pixel value is 64 [at this time, an arearatio of lighted cells (cells with output pixel values other than zero)with respect to all the cell regions in the screen cell 50 is 25%].

Similarly, FIG. 12B shows the dot pattern of the screen cell 50 when theinput pixel value is 68 (area ratio=26.5625%), FIG. 12C shows the dotpattern of the screen cell 50 when the input pixel value is 72 (arearatio=28.125%), FIG. 12D shows the dot pattern of the screen cell 50when the input pixel value is 76 (area ratio=29.6875%), FIG. 12E showsthe dot pattern of the screen cell 50 when the input pixel value is 80(area ratio=31.25%), and FIG. 12F shows the dot pattern of the screencell 50 when the input pixel value is 96 (area ratio=37.5%).

In FIG. 12A through 12F, when the input pixel value is 64, an outputpixel value 255 is read from the lookup table 123 b (see AD64 of FIG.11) for the individual cells with cell numbers 2 and 27, and outputpixel value 0, . . . are read for the individual cells with cell numbers7, 14, 15, 22, 23 and 30. As a result, the screen dot pattern becomes asshown in FIG. 12A when the output pixel values read for all the cellsare disposed at the pertinent cell positions of the screen cell 50.

And, when the input pixel value is 68, the output pixel value read fromthe lookup table 123 b (see AD68 of FIG. 11) is 255 for the cell withthe cell number 2, 128 for the cells with cell numbers 7, 27 and 30, and0, . . . for the individual cells with cell numbers 14, 15, 22 and 23.As a result, the screen dot pattern becomes as shown in FIG. 12B whenthe output pixel values read for all the cells are disposed at thepertinent cell positions.

When the input pixel value is 72, the output pixel value read from thelookup table 123 b (see AD72 of FIG. 11) is 255 for the individual cellswith cell numbers 2, 7 and 30, and 0, . . . for the individual cellswith cell numbers 14, 15, 22, 23 and 27. As a result, the screen dotpattern becomes as shown in FIG. 12C when the output pixel values readfor all the cells are disposed at the pertinent cell positions.

When the input pixel value is 76, the output pixel value read from thelookup table 123 b (see AD76 of FIG. 11) is 128 for the individual cellswith cell numbers 2, 25 and 22, 255 for the individual cells with cellnumbers 7 and 30, and 0, . . . for the individual cells with cellnumbers 14, 23 and 27. As a result, the screen dot pattern becomes asshown in FIG. 12D when the output pixel values read for all the cellsare disposed at the pertinent cell positions.

When the input pixel value is 80, the output pixel value read from thelookup table 123 b (see AD80 of FIG. 11) is 0 for the individual cellswith cell number 2, 14, 23 and 27, and 255, . . . for the individualcells with cell numbers 7, 15, 22 and 30. As a result, the screen dotpattern becomes as shown in FIG. 12E when the output pixel values readfor all the cells are disposed at the pertinent cell positions.

In addition, when the input pixel value is 96, the output pixel valueread from the lookup table 123B (see AD96 of FIG. 11) is 0 for theindividual cells with cell numbers 2 and 27, and 255, . . . for theindividual cells with cell numbers 7, 14, 15, 22, 23 and 30. As aresult, the screen dot pattern becomes as shown in FIG. 12F when theoutput pixel values read for all the cells are disposed at the pertinentcell positions.

Here, the growth of the screen cell dot by use of the lookup table 123 bhaving the data structure shown in FIG. 11 is verified through thecomparison of the individual screen dot patterns of FIG. 12A throughFIG. 12F.

First, a growth process of screen dots when the input pixel value 64changes to the input pixel value 72 will be described with reference tocell numbers 7, 27 and 30.

When the input pixel value=64 [FIG. 12A], the cell with cell number 27is in a fully lighted state, and the individual cells with cell numbers7 and 30 are in a non-lighted state.

Then, with respect to the increase in the input pixel value, the dot ofthe cell with cell number 27 is gradually made small, while the dots ofthe individual cells with cell numbers 7 and 30 are gradually made largeto change the screen shape from the dot type at the time when inputpixel value=64 to a thin line type, while maintaining the same linenumber.

For example, the cell with cell number 27 has an output pixel value 255when input pixel value=64 [FIG. 12A], then the output pixel value isdecreased at 32 intervals in accordance with the increase in the inputpixel value, and the output pixel value is made to 0 (lights out) wheninput pixel value=72 [FIG. 12C].

At the same time, for the cells with cell numbers 7 and 30, the outputpixel value is increased at 32 intervals from input pixel value=64 [FIG.12A] in accordance with the increase in the input pixel value, so thatthe output pixel value is made to have full lighting (255) when inputpixel value=72 [FIG. 12C].

Thus, the individual cells with cell numbers 7, 27 and 30 when inputpixel value=68 in FIG. 12B have a lighted state which is in the middleof changing from input pixel value=64 to input pixel value=72.

As described above, the screen pattern is changed while keeping a smoothincrease in gradation at the time of changing from the input pixelvalue=64 to the input pixel value=72.

Similarly, when changing as indicated by input pixel value=72 [FIG.12C]→input pixel value=76 [FIG. 12D]→input pixel value=80 [FIG. 12E](increase in the input pixel value), the output pixel values of theindividual cells with cell numbers 2 and 38 are decreased, and at thesame time, the output values of the individual cells with cell numbers15, 22, 43 and 50 are increased.

Besides, when changing as indicated by input pixel value=80 [FIG. 12E]→input pixel value=96 [FIG. 12F], the cells with cell numbers 14 and 51are also increased to grow the dots in a line type pattern.

As described above, the dot pattern can be changed from the dot typepattern when input pixel value=64 [FIG. 12A] to the line type patternwhen input pixel value=96 [FIG. 12F]. During the change, the line numberof the screen cell 50 is kept to have the same screen line numberwithout changing.

The dot pattern change described above can be realized by use of thelookup table 123 b in which the data contents shown in FIG. 11 arestored.

In the embodiment described above, the output pixel value correspondingto the input pixel value is stored to increase and decrease thecorresponding output pixel value with the increase in the input pixelvalue. Especially, the lookup table 123 b, in which the output pixelvalues corresponding to the input pixel values are stored to form animage that the screen line number is kept at a prescribed level with theincrease in the input pixel value but the dot positions are different,is used. Therefore, even when the screen has the same line number, thedot pattern shape can be changed freely in accordance with the gradationcharacteristics, and a screen optimum to the properties of a printerengine (image forming section 14) can be designed.

In this embodiment, it is not limited to the case that the inputmultiple gradation image is converted into the multivalue image by useof the area gradation method according to the screen cell, but it isalso possible to convert into a binary image by adding the replace andread processing function for reading and outputting the output pixelvalues 0 to 255, which are set in accordance with the individual screencell numbers by the lookup table 123 b of, for example, FIG. 11, with 0to less than 129 replaced by “0” and 129 or more replaced by “1” in thesame manner as in the first embodiment.

Both the first and second embodiments described above are based on useof one screen cell 50, do not need another screen for each of a smallline number and a large line number or a switching function of eachscreen and can realize the apparatus at a low cost.

By providing the setting portion 121, it is easy to flexibly conform tothe properties of a printer by simply changing the screen cellstructure.

According to the construction of the present invention, it is easy todisplace the dot positions within the screen cell 50, so that moiré andthe like can be avoided, and a change in the delicate pattern can beproduced, so that a gradation jump due to dot gain can also beprevented.

In the first and second embodiments described above, the example ofusing the lookup table 123 to realize a dot growth process that theoutput pixel value is increased or decreased with the increase in theinput pixel value is described, but the dot growth process can also berealized without using the lookup table 123. For example, measures maybe taken to store only a change point and a rate of change in a registerand to make interpolation calculations linearly during that storage.

As described above, a first aspect of the present invention provides animage processing apparatus for converting a multiple gradation imageinto a binary or multivalue image by use of an area gradation methodaccording to a screen cell, which includes: a receiving unit thatreceives an input pixel value of each pixel of the multiple gradationimage and position information on the screen cell; and an output unitthat outputs an output pixel value, which is increased or decreased withan increase in the input pixel value, at a pixel position on the screencell corresponding to the position information, according to the inputpixel value and the position information received by the receiving unit.

A second aspect of the present invention provides the image processingapparatus according to the first aspect of the invention, wherein theoutput unit includes: a lookup table in which the output pixel valuecorresponding to the input pixel value is stored in such a manner thatthe corresponding output pixel value is increased or decreased with theincrease in the input pixel value, and outputs the output pixel value byreferring to the lookup table on the basis of the input pixel value andthe position information received by the receiving unit.

A third aspect of the present invention provides the image processingapparatus according to the second aspect of the invention, wherein thelookup table stores the output pixel value corresponding to the inputpixel value in such a manner that the output pixel value monotonicallyis increased from zero with the increase in the input pixel value, thenreturned to zero once and monotonically increased again.

A fourth aspect of the present invention provides the image processingapparatus according to the second aspect of the invention, wherein thelookup table stores the output pixel value corresponding to the inputpixel value in such a manner that a screen line number is continuouslychanged with the increase in the input pixel value.

A fifth aspect of the present invention provides the image processingapparatus according to the second aspect of the invention, wherein thelookup table stores the output pixel value corresponding to the inputpixel value in such a manner that an image is formed in which a screenline number is kept at a prescribed level with the increase in the inputpixel value but a dot position is different.

A sixth aspect of the present invention provides an image processingmethod which converts a multiple gradation image into a binary ormultivalue image by use of an area gradation method according to ascreen cell, which includes: receiving an input pixel value of eachpixel of the multiple gradation image and position information on thescreen cell; and outputting an output pixel value which is increased ordecreased with an increase in the input pixel value at a pixel positionon the screen cell corresponding to the position information, accordingto the received input pixel value and the position information.

A seventh aspect of the present invention provides the image processingmethod according to the sixth aspect of the invention, wherein: thecorresponding output pixel value which is increased and decreased withthe increase in the input pixel value is previously stored at each ofpixel positions of the screen cell on a lookup table in which an outputpixel value corresponding to an input pixel value of each pixel of themultiple gradation image is stored; and an output pixel value is outputbased on the input pixel value and the position information by referringto the lookup table.

An eighth aspect of the present invention provides the image processingmethod according to the sixth aspect of the invention, wherein: thelookup table stores the output pixel value corresponding to the inputpixel value in such a manner that the output pixel value ismonotonically increased from zero with the increase in the input pixelvalue, then returned to zero once and monotonically increased again; anda single lookup table is used to form an image having a different screenline number between a low gradation portion and a high gradationportion.

A ninth aspect of the present invention provides the image processingmethod according to the sixth aspect of the invention, wherein: thelookup table stores the output pixel value corresponding to the inputpixel value in such a manner that a screen line number is continuouslychanged with the increase in the input pixel value; and a single lookuptable is used to form the image having a screen line number continuouslychanging with the increase in the input pixel value.

A tenth aspect of the present invention provides the image processingmethod according to the sixth aspect of the invention, wherein: thelookup table stores the output pixel value corresponding to the inputpixel value in such a manner that an image is formed in which a screenline number is kept at a prescribed level with the increase in the inputpixel value but a dot position is different; and a single lookup tableis used to form the image having a screen line number kept at theprescribed level according to the increase in the input pixel value buta different dot position.

According to the present invention, the input pixel values of individualpixels of a multiple gradation image and the position information on thescreen cells are accepted, and according to the accepted input pixelvalues and position information, the output pixel values which increaseor decrease with the increase in the above-described input pixel valuesare output in correspondence with the pixel positions on the screencells corresponding to the above-described position information.Therefore, the dot growth process capable of changing the screenstructure and screen line number by use of one screen cell can berealized, stable reproducibility can be obtained in a low gradation areaby a small line number screen, and improvement of definition inintermediate and high gradation areas can be expected.

Because one screen cell is used in the present invention, it is notnecessary to provide a different screen for each of the small linenumber and the large line number or to provide a switching function ofeach screen, and the apparatus can be realized at a low cost.

Besides, it is also possible to flexibly conform to the properties of aprinter by simply changing the screen cell structure according tosettings.

The present invention can be applied to an image processing apparatusthat a multiple gradation image is converted to a binary or multivalueimage by use of an area gradation method according to a screen cell andcan use a lookup table in which output pixel values corresponding toinput pixel values are stored so to increase or decrease a correspondingoutput pixel value with an increase in the input pixel value to switch ascreen structure flexibly without requiring plural screen cells and aswitching function thereof.

The foregoing description of the embodiments of the present inventionhas been provided for the purpose of illustration and description. It isnot intended to be exhaustive or to limit the invention to the preciseforms disclosed. Obviously, many modifications and variations will beapparent to practitioners skilled in the art. The embodiments werechosen and described in order to best explain the principles of theinvention and its practical applications, thereby enabling other skilledin the art to understand the invention for various embodiments and withthe various modifications as are suited to the particular usecontemplated. It is intended that the scope of the invention be definedby the following claims and their equivalents.

The entire disclosure of Japanese Patent Application No. 2005-93910filed on Mar. 29, 2005 including specification, claims, drawings andabstract is incorporated herein by reference in its entirety.

1. An image processing apparatus for converting a multiple gradationimage into a binary or multivalue image by use of an area gradationmethod according to a screen cell, comprising: a receiving unit thatreceives an input pixel value of each pixel of the multiple gradationimage and position information on the screen cell; and an output unitthat outputs an output pixel value, which is increased or decreased withan increase in the input pixel value, at a pixel position on the screencell corresponding to the position information, according to the inputpixel value and the position information received by the receiving unit.2. The image processing apparatus according to claim 1, wherein: theoutput unit comprises: a lookup table in which the output pixel valuecorresponding to the input pixel value is stored in such a manner thatthe corresponding output pixel value is increased or decreased with theincrease in the input pixel value, and outputs the output pixel value byreferring to the lookup table on the basis of the input pixel value andthe position information received by the receiving unit.
 3. The imageprocessing apparatus according to claim 2, wherein: the lookup tablestores the output pixel value corresponding to the input pixel value insuch a manner that the output pixel value monotonically is increasedfrom zero with the increase in the input pixel value, then returned tozero once and monotonically increased again.
 4. The image processingapparatus according to claim 2, wherein: the lookup table stores theoutput pixel value corresponding to the input pixel value in such amanner that a screen line number is continuously changed with theincrease in the input pixel value.
 5. The image processing apparatusaccording to claim 2, wherein: the lookup table stores the output pixelvalue corresponding to the input pixel value in such a manner that animage is formed in which a screen line number is kept at a prescribedlevel with the increase in the input pixel value but a dot position isdifferent.
 6. An image processing method which converts a multiplegradation image into a binary or multivalue image by use of an areagradation method according to a screen cell, comprising: receiving aninput pixel value of each pixel of the multiple gradation image andposition information on the screen cell; and outputting an output pixelvalue which is increased or decreased with an increase in the inputpixel value at a pixel position on the screen cell corresponding to theposition information, according to the received input pixel value andthe position information.
 7. The image processing method according toclaim 6, wherein: the corresponding output pixel value which isincreased and decreased with the increase in the input pixel value ispreviously stored at each of pixel positions of the screen cell on alookup table in which an output pixel value corresponding to an inputpixel value of each pixel of the multiple gradation image is stored; andan output pixel value is output based on the input pixel value and theposition information by referring to the lookup table.
 8. The imageprocessing method according to claim 6, wherein: the lookup table storesthe output pixel value corresponding to the input pixel value in such amanner that the output pixel value is monotonically increased from zerowith the increase in the input pixel value, then returned to zero onceand monotonically increased again; and a single lookup table is used toform an image having a different screen line number between a lowgradation portion and a high gradation portion.
 9. The image processingmethod according to claim 6, wherein: the lookup table stores the outputpixel value corresponding to the input pixel value in such a manner thata screen line number is continuously changed with the increase in theinput pixel value; and a single lookup table is used to form the imagehaving a screen line number continuously changing according to theincrease in the input pixel value.
 10. The image processing methodaccording to claim 6, wherein: the lookup table stores the output pixelvalue corresponding to the input pixel value in such a manner that animage is formed in which a screen line number is kept at a prescribedlevel with the increase in the input pixel value but a dot position isdifferent; and a single lookup table is used to form the image having ascreen line number kept at the prescribed level with the increase in theinput pixel value but a different dot position.